This invention relates to a serial impact printer and, in particular, to a serial impact printer having a plurality of tension rollers which apply tension to a sheet of paper to be printed and a sheet guide which reduces the volume of noise produced by the printer.
A conventional serial impact printer 100 constructed in accordance with the prior art is shown in FIG. 1. Serial impact printer 100 is provided with a platen tension rollers 12 (only one shown), and sheet retaining rollers 13, 14 and 15. Platen 11 and tension rollers 12 are rotated in the directions of arrows A and B, respectively. The rotation of tension rollers 12 cause printing sheet P to travel in the direction of arrow C. A sheet retaining plate 16 is adapted to prevent a printing sheet P from being lifted away from platen 11 as it is conveyed through a printing region. The sheet is further guided past an impact dot head 20 and a carriage 21 on which head 20 is mounted. A sheet guide 17 and discharging sheet guides 18 and 19 are provided for guiding the front end portion of a printed sheet towards tension rollers 12. Carriage 21 is guided along guide shafts 22 and 23 to vertically move print head 20 across the printing sheet resting against platen 11.
Further, a guide member 24 is mounted on carriage 21. Guide member 14 is adapted to guide printing sheet P along the outer cylindrical wall of platen 11 at the printing region where platen 11 confronts print head 20. In order to guide sheet P along platen 11 towards tension rollers 12, guide member 24 is shaped with a small gap formed between the guide surface of the guide member 24 and the outer cylindrical wall of platen 11. In this manner, a tangent line extending from platen 11 to tension rollers 12 is formed representing the path of sheet P.
In serial impact printer 100, the peripheral speed of tension rollers 12 is rotated at a speed slightly greater than the peripheral speed of platen In this manner, the conveyance of sheet P by platen 11 is assisted by tension rollers 12. Further, a tension is applied to sheet P to prevent sheet P from being lifted from platen 11. The application of tension also reduces the volume of noise produced during printing. Although only one tension roller 12 is shown in FIG. 1, a plurality of tension rollers are arranged in the direction of the axis of platen 11. During the printing operation, guide member 24 is not in contact with sheet P as shown in FIG. 1.
The following problems are presented by the construction of the conventional serial impact printer described above. In a typical printing environment, sheets four to sixteen inches in length are used. As set forth above, a plurality of tension rollers 12 are arranged in the direction of the axis of platen 11. Hence, the tension is distributed in the width direction of sheet P. In this configuration, a suitable tension is applied to the section of the sheet confronting tension rollers 12. However, the tension applied to the section of the sheet not confronting tension rollers 12 is unsuitable. Therefore, this section of sheet P is lifted from platen 11 making it difficult to prevent the production of noises during the printing operation.
In order to uniformly distribute the tension over the sheet, a longer tension roller may be employed such that the roller is as long as platen 11. However, such a construction is also disadvantageous in that the longer components increase the manufacturing costs. Accordingly, it is difficult to reduce the manufacturing cost of the printer with a construction of this type. Since it is essential to polish the outer cylindrical wall of the tension roller, it is impossible to reduce the diameter of the tension roller and thereby miniaturize it, while maintaining a low manufacturing cost.
Alternatively, a sheet conveyance force of tension rollers 12 could be increased to maintain a suitable tension on the sheet of paper. However, since the peripheral speed of tension rollers 12 is greater than that of platen 11, sheet P slips on platen 11. Therefore, it is difficult to convey the sheet through the printer with high accuracy. When a continuous form sheet is used in conjunction with a push type tractor assembly, the printer is operated with sheet retaining rollers 13 and 14 r.RTM.1.RTM.as.RTM.d. Therefore, sheet P is more likely to slip on platen 11, and therefore may improperly travel over the tractor pins.
Accordingly, it is desired to provide a serial impact printer which can be manufactured at a relatively low cost which is miniaturized, and which conveys printing sheets with high printing accuracy while producing less noise.